Time–frequency analysis of transient evoked-otoacoustic emissions in individuals with auditory neuropathy spectrum disorder

Hearing Research 313 (2014) 1e8

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Research paper

Timeefrequency analysis of transient evoked-otoacoustic emissions in individuals with auditory neuropathy spectrum disorder Vijaya Kumar Narne, P. Prashanth Prabhu*, Suma Chatni Department of Audiology, All India Institute of Speech and Hearing, Mysore, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 November 2013 Received in revised form 8 April 2014 Accepted 15 April 2014 Available online 24 April 2014

The aim of the study was to describe and quantify the cochlear active mechanisms in individuals with Auditory Neuropathy Spectrum Disorders (ANSD). Transient Evoked Otoacoustic Emissions (TEOAEs) were recorded in 15 individuals with ANSD and 22 individuals with normal hearing. TEOAEs were analyzed by Wavelet transform method to describe and quantify the characteristics of TEOAEs in narrowband frequency regions. It was noted that the amplitude of TEOAEs was higher and latency slightly shorter in individuals with ANSD compared to normal hearing individuals at low and mid frequencies. The increased amplitude and reduced latencies of TEOAEs in ANSD group could be attributed to the efferent system damage, especially at low and mid frequencies seen in individuals with ANSD. Thus, wavelet analysis of TEOAEs proves to be another important tool to understand the patho-physiology in individuals with ANSD. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Auditory Neuropathy Spectrum Disorder (ANSD) is a condition in which an individual presents with normal oto-acoustic emissions and absent/abnormal Auditory Brainstem Response (ABR) (Starr et al., 1996; Berlin et al., 2003). The site of dysfunction in ANSD is presumed to be either the inner hair cells or at the junction of the spiral ganglion cells and/or in the auditory nerve (Nachman, 2012; Amatuzzi et al., 2011). In individuals with ANSD, the puretone thresholds may range anywhere from normal hearing sensitivity to a profound hearing loss (Berlin et al., 2010). Speech recognition ability is generally poor and disproportionate to the pure tone average (Rance, 2005). Otoacoustic emissions (OAEs) are one of the important diagnostic indicators for ANSD (Berlin et al., 2010). Transient evoked otoacoustic emissions (TEOAEs) are reported to be present in 80% of individuals with ANSD (Starr et al., 2000; Berlin et al., 2010). Thus, TEOAEs are commonly employed as a clinical tool for examining cochlear functioning in individuals with ANSD (Hood and Berlin, 2001; Kumar and Jayaram, 2006; Berlin et al., 2010). In addition to the above observations, Hood and Berlin (2001) have noted that the amplitude of TEOAEs in individuals with ANSD is abnormally higher compared to individuals with normal hearing. The high * Corresponding author. Department of Audiology, All India Institute of Speech and Hearing, Naimisham Campus, Manasagangothri, Mysore, Karnataka 570006, India. Tel.: þ91 8904353390. E-mail address: [email protected] (P.P. Prabhu). http://dx.doi.org/10.1016/j.heares.2014.04.005 0378-5955/Ó 2014 Elsevier B.V. All rights reserved.

amplitude has been attributed to lack of efferent suppression of otoacoustic emissions in these listeners (Hood and Berlin, 2001). Following these observations, Avilala et al. (2012) examined spontaneous otoacoustic emissions (SOAEs) in individuals with ANSD. They noted that the occurrence of SOAEs was more at frequencies lower than 1.5 kHz in individuals with ANSD compared to individuals with normal hearing. Further, they also observed that more number of individuals with ANSD had presence of SOAEs at multiple frequencies than individuals with normal hearing. The results of the above studies suggest that individuals with ANSD may have a subtle auditory dysfunction at the level of the outer hair cells (OHCs) responsible for the cochlear active mechanism. Thus, to investigate the functionality of the cochlear active mechanism in individuals with ANSD, TEOAEs, which measure the functionality of OHCs, were recorded and analyzed. In order to improve the sensitivity of TEOAEs in detecting subtle auditory dysfunction, TEOAEs were subjected to timeefrequency analysis in addition to the conventional Fast Fourier Transform (FFT) analysis. The TEOAE waveform is the sum of evoked responses from different cochlear locations with each location tuned to a specific frequency. The latency of the cochlear response is different from the base to the apex of the cochlea and each specific frequency contributing to the overall response, producing a time-varying TEOAE waveform. The conventional FFT analysis is appropriate for stationary waveforms but not for the time varying waveform. FFT analysis may not capture the subtle changes in the TEOAE waveform which are direct consequences of the time-varying behavior of TEOAE signal. Hence,

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FFT analysis may not be accurate in all the cases, where it is necessary to extract parameters that may reveal subtle and subclinical alterations of the signal. Time frequency analysis is thus essential to examine the cochlear functionality in the different narrow band regions of the basilar membrane that are tuned to different characteristic frequencies (Tognola et al., 1997). Timeefrequency analysis of TEOAEs can be performed using Wavelet Transform (WT) (Mallat, 1989), which extracts frequency components in narrow band regions. In wavelet transform approaches, the time and frequency resolution is not fixed over the entire timeefrequency axis but can vary. Because of this, high-frequency components can be analyzed with good time resolution, while low-frequency components can be analyzed with good frequency resolution. In particular, the WT analysis gives very accurate results if the signal is made up of low-frequency components of long duration and high-frequency components of brief duration, such as in the case of TEOAEs. Tognola et al. (1998) has proposed a procedure of WT analysis, which is particularly suitable for analysis of TEOAEs. This algorithm has been extensively used for time frequency analysis of TEOAEs to understand the cochlear mechanisms in newborns and young infants (Tognola et al., 2001; Moleti et al., 2005), in individuals with noise-induced hearing loss and tinnitus (Tognola et al., 1999; Paglialonga et al., 2011), the effects of exposure to electromagnetic fields on cochlear mechanisms in adults (Paglialonga et al., 2007), and in children exposed to carboplatin chemotherapy (Bhagat et al., 2013). All the previous studies in ANSD, typically measured the amplitude of TEOAEs in half-octave band regions or broadband energy. However, there is a dearth of studies which have investigated the functionality of cochlear active mechanisms in individuals with ANSD using timeefrequency analysis of TEOAEs. The time frequency analysis would allow us to examine the energy of TEOAEs in the narrow-band frequency regions and their corresponding latencies. The amplitude and latency of TEOAEs measured at narrow-band frequencies are sensitive to subtle OHC dysfunction (Moleti et al., 2005). Hence, the present study aimed to examine the energy and latency of TEOAEs in individuals with ANSD using time frequency analysis of TEOAEs. 2. Material and methods 2.1. Participants

The individuals in the ANSD group had an average pure-tone thresholds (average of 0.5. 1, 2, 4 kHz) ranging from 36 dB HL to 50 dB HL with symmetrical sensorineural hearing loss. Fig. 1 shows the pure-tone thresholds of normal hearing group and ANSD group across the octave frequencies ranging from 0.25 to 8 kHz. These individuals had normal tympanometric findings with absent ipsilateral and contralateral acoustic reflexes. An otological evaluation was also performed to rule out any middle ear disorders. All individuals in the ANSD group had robust TEOAEs suggestive of preserved outer hair-cell functioning and absent or severely abnormal auditory brainstem responses indicative of disordered auditory nerve functioning as the basis for the diagnosis of ANSD. A neurologist confirmed the diagnosis with a detailed clinical neurological examination, which also included CT/MRI tests. None of the individuals used any hearing aid or assistive listening device. The details of the individuals in the ANSD group are shown in Table 1. 2.2. Procedure A calibrated dual channel diagnostic clinical audiometer was used for estimating the pure-tone air-conduction thresholds, bone conduction thresholds using Modified Hughson and Westlake procedure (Carhart and Jerger, 1959) and speech identification scores. A calibrated immittance meter GSI-Tympstar was used to assess middle ear status. Auditory Brainstem Response (ABR) was measured, at varying intensities starting from 90 dB nHL, with the Intelligent Hearing System (IHS)/Biologic EP fitted with an ER-3A insert earphone. Speech identification testing was done with the monitored live voice presentation of phonemically balanced words in Kannada (Yathiraj and Vijayalakshmi, 2005) at 40 dB SL (re: Speech Recognition Threshold). 2.2.1. Recording of TEOAEs TEOAEs were recorded from all the individuals of both groups seated in a sound treated room, using Otodynamics ILO-292 system. TEOAEs were acquired using ‘non-linear’ protocol with two trains of click stimuli (80ms rectangular clicks) at 75e80 dB peak SPL. The response to the click trains were stored and averaged in two 20 ms buffers A and B. A total of 260 click trains (260*4 ¼ 1040 clicks) were presented at a rate of 50/s for each measurement run. To ensure high reliability of TEOAE recordings and for investigating minute changes in auditory function, only measurements with

The participants of the study were divided into two groups.

2.1.2. Group-2: ANSD group The auditory neuropathy spectrum disorder (ANSD) group consisted of 15 individuals (6 males & 9 females) with late diagnosis1 of ANSD in the age range of 13e25 years with a mean age of 17.7 years.

0 10

Intensity (dB HL)

2.1.1. Group-1: normal hearing group The normal hearing group (NH) consisted of 22 individuals (10 males & 12 females) in the age range of 17e26 years with a mean age of 20.8 years. All the individuals had pure-tone thresholds lesser than 15 dB HL at octave frequencies and speech identification scores of 100% in quiet at 40 dB SL (re: Speech Recognition Threshold) on routine speech audiometry. They also had normal tympanometric results with both ipsilateral and contralateral reflexes present.

-10

20 30 40 50 60

ANSD Normal Hearing

70 80

1

The term late diagnosis was used, owing to the lack of information regarding the onset of the condition in individuals with ANSD. Some of the individuals in the ANSD group may have had congenital ANSD but may have started to consider their hearing difficulties later in life, when the listening environment became more complex. However, the others might have acquired the ANSD condition.

125

250

500

1000

2000

4000

8000

Frequency (Hz) Fig. 1. The mean pure-tone threshold (1 SD) as a function of frequency for individuals with normal hearing and individuals with ANSD.

V.K. Narne et al. / Hearing Research 313 (2014) 1e8 Table 1 Demographic details and audiometric results of ANSD group. Name

Subject Subject Subject Subject Subject Subject Subject Subject Subject Subject Subject Subject Subject Subject Subject

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Age (in years)

Gender

18 19 21 19 16 15 14 14 13 12 14 24 19 25 22

Male Female Female Male Female Female Male Female Female Male Female Male Female Male Female

PTA

SIS (%)

Right ear

Left ear

Right ear

Left ear

41.66 48.33 48.33 36.66 50 45 48.66 41.66 40 45 41.66 45 50 41.66 50

41.66 41.66 41.66 43.33 50 48.33 41.66 46.66 43.33 45 46.66 46.66 45 43.33 46.66

60 52 64 52 24 44 40 52 36 20 36 36 32 64 44

64 48 48 56 32 64 64 36 48 16 44 52 32 44 32

reproducibility greater than 60% and high emission levels above the noise floor with overall SNR of 6 dB were considered (Groh et al., 2006; Hall, 2000). Along with the above, stable probe placement in the ear was taken care of by monitoring the percent stimulus stability (i.e. an estimate of average correlation between the waveforms of the stimulus at the beginning & at the end of each recording) to be higher than 90% throughout the whole recording session. All the individuals in both the groups completed the recording procedure of TEOAEs and had the SNR criteria as stated above.

2.3. Wavelet transform of TEOAEs TEOAE waveform is a complex signal obtained by the summation of evoked response originated from different regions in the cochlea (Kemp, 1978). The temporal ordering of frequencies in the TEOAE waveform is similar to the frequency and place mapping in the cochlea where higher frequencies appear first and low frequencies appear later. Among the time frequency analysis methods, WT analysis provides very sharp frequency and time resolution when compared to other methods (Jedrzejczak et al., 2005; Marozas et al., 2006; Tognola et al., 1997, 1998; Yang et al., 2002; Zhang et al., 2008). WT analysis divides the complex TEOAE waveforms into various frequency components at the generic time t and frequency f0 of the TEOAEs signal x (t) using the following formula (Mallat, 1989).

WTf ðsÞ ¼

t

sffiffiffiffi   f * f xðtÞ$ $ðt  sÞ dt $j f0 f0

(1)

where the function * is a complex conjugation. This WT decomposes the signal x (t)ffi into elementary components by a bank of pffiffiffiffiffiffiffiffiffiffiffiffi band pass filters ðf =f0 Þ$j* ðf =f0 $ðtÞÞ; which are iteratively derived from a function called the mother wavelet (W (t)). The mother wavelet is a function which has finite energy at t ¼ 0; and acts as a band-pass filter centered at f0 in frequency domain. The function for generation of mother wavelet used in the study is similar to that proposed by Tognola et al., (1998) which is given in Equation (2).

Wt ¼

1  cosð20tÞ; where ðn ¼ 2Þ ð1 þ t*2n Þ

95.44 Hz wide in the frequency range of 500e5000 Hz. Fig. 2 shows a subset of the 48 frequency components extracted from TEOAEs in 8 individuals, 4 from the normal hearing group and 4 from the ANSD group. Broadband TEOAEs of both the individuals are shown in the top row of the figure and the following 6 rows represent narrow-band frequency components centered at 1, 1.5, 2, 2.5, 3, and 4 kHz extracted from TEOAEs in the top row. 2.3.1. TEOAEs quantitative parameters The amplitude and latency of TEOAEs were evaluated for each waveform. The extracted parameters were compared between the normal hearing group and the ANSD group.

Note: All participants had absent acoustic reflexes, present TEOAEs and absent ABR at 90 dB nHL.

Z

3

(2)

TEOAEs were decomposed, using equations (1) and (2) into 48 frequency components with each frequency component being

2.3.1.1. Amplitude. The amplitude is the root mean square energy (rms) of both broadband and individual frequency components of TEOAEs. The amplitude of the broadband TEOAEs was measured as the root mean square (rms) value in micro-Pascals (mPa) and presented in dB SPL. For the broadband TEOAEs, signal (Sx) was estimated by measuring the average of the two replicated recordings of TEOAEs, Sa and Sb, stored in the two buffers A and B.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n   u1 X Ai þ Bi 2 Am ¼ t n i 2

(3)

Similarly, the amplitude (rms value) of each of the 48 frequency components extracted using WT analysis was computed using the same Eq. (Berlin et al., 1993) by substituting Sa and Sb with the components extracted, at each frequency, from each of the two TEOAE recording buffers. 2.3.1.2. Latency. In addition to Amplitude (AMP), Latency (LAT) of 48 narrow band frequency components of TEOAEs was used to evaluate the quantitative effect of ANSD on frequency specific TEOAEs. The latency of the narrow band frequency components was defined as the time interval from the stimulus onset to the maximum of the temporal envelope of the waveform (Tognola et al., 1998). The analysis of noisy narrow band components was avoided by calculating the percent reproducibility between A and B replicates of narrow band components of TEOAEs. The bands with reproducibility of 50% were included for analysis and those with <50% reproducibility were discarded. As a result of this, the number of individuals selected for the further analysis varied from 11 to 13 (as a function of the frequency band). 2.4. Ethical considerations In the present study, all the testing procedures were done using non-invasive techniques adhering to conditions of ethical approval committee of All India Institute of Speech and Hearing, Mysore. All the test procedures were explained to the patients and their family members before testing and informed consent was taken from all the patients or their family members for participating in the study. 2.5. Statistical analysis AndersoneDarling test and ShapiroeWilk test of normality were performed on TEOAE quantitative parameters (i.e. AMP & LAT). Analysis revealed that AMP data were normally distributed across most of the frequencies, except at 6 frequencies in both groups. The LAT data was skewed at 12 frequencies, so square root transformation was applied. After the transformation, the test for normality indicated that latency was normally distributed at most of the frequencies except at 4 frequencies. The differences in the mean AMP of broadband TEOAEs and noise level between the two

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Fig. 2. Click evoked TEOAEs with wavelet derived narrow band components for four normal hearing and four ANSD individuals are shown. The top row shows the original TEOAE response and five other rows show wavelet derived components at 1, 1.5, 2, 2.5, 3 and 4 kHz.

groups were assessed using independent sample t-test. Multivariate repeated measures of analysis of variance was used to assess the effects of group, frequency, and their interaction on mean AMP and mean LAT of the narrow-band frequency components of TEOAEs. In all the statistical analyses performed, p value lower than 0.05 was considered as significant. Eta square (h2) was computed to represent the population effect size. 3. Results

amplitude of TEOAEs was lesser in ANSD group compared to the normal hearing group. The statistical analysis revealed a significant effect of group on the mean amplitude [F(1,242) ¼ 6.17, p < 0.05] indicating that mean amplitude was higher in ANSD group compared to the normal hearing group. There was a significant interaction between group and frequency [F(47,7.9) ¼ 3.99, p < 0.01]. Bonferroni’s post-hoc multiple comparisons test indicated that the observed difference in amplitude between ANSD and normal hearing group reached significance (h2 > 0.65) in 18 frequency band components in the frequency range of 0.7e2.4 kHz. But the

3.1. Amplitude 3.1.1. Broadband TEOAEs The mean amplitude and noise levels of broadband TEOAEs are shown in Fig. 3. The difference in mean amplitude between groups was compared with t-test and the analysis showed that the mean amplitude was higher in ANSD group compared to the normal hearing group (p < 0.05; h2 > 0.6). However, the mean difference in noise levels between the two groups did not reach significance, indicating that TEOAEs recordings in ANSD and normal hearing group had a similar noise floor. 3.1.2. Narrow band TEOAEs The mean energy across 48 frequency bands for both the groups is shown in Fig. 4. The figure shows that the mean energy of ANSD group was higher at lower frequencies compared to the normal hearing group. In addition, at frequencies higher than 4 kHz, mean

Fig. 3. The mean (1 SD) of amplitude and noise level (dB SPL) of broadband TEOAEs obtained in normal hearing, ANSD and Older ANSD groups. The ANSD group included all the 15 individuals with ANSD and Older ANSD group included 9 individuals with age >16 years.

V.K. Narne et al. / Hearing Research 313 (2014) 1e8

25

Amplitude(dB SPL)

20 15 10 5 0

Normal Hearing ANSD Older ANSD

-5 -10 -15

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Frequency (Hz) Fig. 4. The mean (1 SD) amplitude of TEOAEs as a function of frequencies for normal hearing (open circle), ANSD (open square) and Older ANSD (open triangle) groups. Filled symbols indicate the frequencies at which the mean amplitudes of ANSD (filled squares) and Older ANSD (filled triangles) were significantly (p < 0.05) higher than normal hearing group.

mean difference in amplitude between the groups did not reach significance at high frequencies (>3 kHz), probably because of large standard deviation observed in ANSD group. 3.2. Latency The mean latency with a standard deviation as an error bar for 48 frequency components of TEOAEs measured in both the groups is shown in Fig. 5, which indicates that high frequencies have shorter latency than lower frequencies in both groups. However, the mean latency is shorter in ANSD group (open square) compared to normal hearing group (open circle) at frequencies <2 kHz. As in the previous studies, the latency data in the present study obtained from both the groups was fitted with power-law function (Tognola et al., 1998).

L ¼ a  fb

Fig. 5. The mean (1SD) latencies as a function of frequencies for normal hearing (open circle), ANSD (square) and Older ANSD (open triangle) groups. Filled symbols indicate the frequencies at which the differences in mean latency of ANSD (filled squares) and Older ANSD (filled triangles) were significantly (p < 0.01) lower than normal hearing group. The lines that best fit the mean values of latency in the individuals with normal hearing (gray line) and in the individuals with ANSD (black line) are also shown. The correlation coefficient ‘r’, the slope b, and the p-value of the regression fit are given in the boxes for both groups.

5

Where L and f are the latency (ms) and the frequency (Hz) of the TEOAE components respectively; a and b are constant parameters. The power law function which was fitted is shown in Fig. 5 with thicker lines (black line: ANSD group and gray line: normal hearing group). In both the groups, fitted model was good with r2 > 0.85. The parameter b varied from 0.35 to 0.43 (Mean ¼ 0.401) in ANSD group, whereas in normal hearing group, b varied from 0.41 to 0.50 (Mean ¼ 0.45). The parameter b obtained for normal hearing group was slightly higher than that obtained by previous investigators (Tognola et al., 1997). This higher value observed in the present study may be because of the higher level (80 dB peak SPL) with which the TEOAEs were elicited. The latency data was statistically analyzed and the results revealed a significant effect of group on latency [F(1,237) ¼ 5.3, p ¼ 0.03] and also a significant interaction between group and frequency [F(47,11.5) ¼ 5.4, p ¼ 0.001], suggesting that change in mean latency with frequency was not similar between the groups. Bonferroni’s post-hoc analysis of multiple comparisons revealed that the mean latency was shorter in the ANSD group compared to normal hearing group at low and mid frequencies (<1.5 kHz) with medium-to-large effect size (p < 0.05; h2 > 0.5). 3.3. Influence of age Age is known to greatly influence the characteristics of TEOAEs (Groh et al., 2006; Satoh et al., 1998). In the present study, the ANSD group included six individuals with <16 years of age, whereas, the normal hearing group comprised of individuals with age >17 years. Those six individuals (AN6 to AN11) who were <16 years of age were excluded from the ANSD group and the remaining nine individuals formed a new group, termed hereafter as older ANSD group. The mean broad-band amplitude, narrow-band amplitude and latency of the older ANSD group are shown in Figs. 3e5 respectively. From the figures it can be observed that mean values were changed by a negligible amount after the exclusion of the younger individuals from the ANSD group. In order to assess whether this change would have influenced the group differences, TEOAE broad-band amplitude, narrow-band amplitude and latency data were compared between the normal hearing group and the older ANSD group using statistical tests. A t-test was performed between the normal hearing group and the older ANSD group comparing the broad-band amplitude. The results showed that even after the removal of the younger individuals from ANSD group, the mean difference in amplitude of broad-band TEOAE was statistically significant (t ¼ 2.5, p < 0.01; h2 > 0.5), indicating that the mean broad-band amplitude was minimally influenced by age. Similarly, the narrow band amplitude and latency data were compared between the normal hearing and the older ANSD groups using a multivariate repeated measures of analysis of variance. Results of the narrow band amplitude analysis revealed a significant main effect of group on amplitude [F(1,446) ¼ 5.24, p < 0.05] indicating that mean amplitude was significantly higher in older ANSD group compared to the normal hearing group. Also, a significant interaction between group and frequency [F(47,23) ¼ 2.98, p < 0.01] was observed. Bonferroni’s post-hoc multiple comparisons test indicated that the observed difference in amplitude between older ANSD and normal hearing group reached significance (h2 > 0.58) in 16 frequency band components in the frequency range of 0.7e2.1 kHz. Analysis on the latency data indicated a significant main effect of group on latency [F(1,125.9) ¼ 19.2, p < 0.01] indicating that mean latency was lower in the older ANSD group compared to the normal hearing group. There was also a significant interaction between group and frequency [F(47,1.9) ¼ 1.94, p ¼ 0.001]. Bonferroni’s post-

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hoc multiple comparisons test indicated that the observed difference in latency between older ANSD and normal hearing group reached significance (h2 > 0.53) at frequencies lower than 1.5 kHz. Collectively, these results indicate that the age difference between the normal hearing group and the ANSD group did not contribute significantly to the group differences observed. 4. Discussion 4.1. Amplitude As per the results, the overall amplitude of TEOAEs was higher in ANSD group compared to normal hearing group. The observed difference in mean amplitude between the groups was similar to that reported by Hood and Berlin (2001). Specifically, narrow band frequency analysis of TEOAEs showed that the mean amplitude was significantly higher at low and mid frequencies in ANSD group compared to the normal hearing group. At frequencies higher than 4 kHz, narrow-band TEOAE amplitude of ANSD group was similar or slightly lower than the normal hearing group, which did not reach significance, possibly owing to large individual variability. This variability might be due to the difference in the duration of the ANSD condition across individuals (Deltenre et al., 1999; Shoup and Rosser, 2007). However, this effect was not controlled in the present study. In summary, the amplitude difference between the groups is majorly contributed by the low and mid frequencies. This increase in TEOAE amplitude in ANSD group may be contributed by the spontaneous otoacoustic emissions (SOAE). SOAEs are proven to enhance the amplitude of TEOAEs, both in individuals with normal hearing and those with efferent system dysfunction, such as that noted in individuals with ANSD (Hood and Berlin, 2001), such that the ears with SOAEs, have higher amplitude of TEOAEs (Lonsbury Martin et al., 1991; Prieve and Falter, 1995; Xu et al., 2002). Avilala et al. (2012) have not only shown increased prevalence of SOAEs in ANSD individuals, but also higher SOAE occurrence at low and mid frequencies (<1.5 kHz). Hence, the presence of multiple SOAEs with larger amplitude in individuals with ANSD may get added to the TEOAEs leading to high amplitude TEOAEs at low and mid frequencies. 4.2. Latency Latency of TEOAEs in both the groups followed similar pattern, where high frequencies had shorter latency and low frequencies had longer latencies. This observation is in accordance with the previous studies (Paglialonga et al., 2007; Bhagat et al., 2013). In ANSD group, mean latencies at low and mid frequencies (<1.5 kHz) were shorter compared to the normal hearing group, whereas, at higher frequencies, mean latencies were comparable between the two groups. This group difference in mean latencies at low and mid frequencies can be speculated to be due to a sustained activity in ANSD group, which was observed only at low and mid frequencies and absent at high frequencies (or when observed, it was not as strong as that observed for low frequencies) and was not evident in normal hearing group. Fig. 2 displays the sustained activity in ANSD group which prevails for the entire duration of the acquisition window. This sustained activity may be due to the presence of SOAEs, which are most common among individuals with ANSD and are stronger at low and mid frequencies [<1.5 kHz] (Avilala et al., 2012; Xu et al., 2002). In support to the previous speculations, the prevalence of SOAE in normal hearing group, is in between 1 and 2 kHz (Hall, 2000). This explains the absence of sustained activity at low and mid frequencies in normal hearing group. The exact reason for higher amplitude and sustained activity of TEOAE at lower frequencies in individuals with ANSD compared to

normal hearing is not known. Some of the previous investigators have hypothesized that, there may be an enhancement of cochlear amplification because of decreased efferent system activity which is leading to a certain degree of negative damping of the basilar membrane motion in individuals with ANSD (Hood and Berlin, 2001; Santarelli et al., 2006; Berlin et al., 1998; Santarelli and Arslan, 2002). They proposed that this may explain high amplitude TEOAE, long ringing cochlear microphones (CM) and reduced or absent suppression of TEOAEs observed in individuals with ANSD (Starr et al., 2001; Berlin et al., 1993; Santarelli and Arslan, 2002). The higher amplitude and sustained activity observed at low and mid frequencies could in some way result from reduced effect of efferent system activity at low and mid frequencies. This hypothesis is supported by the observations that effect of efferent system is more prominent at low and mid frequencies (Veuillet et al., 1991; Berlin et al., 1993). As a final remark, it also seems appropriate to discuss the possible effects of age and the level of the stimulus used to evoke TEOAEs, which may have masked or overestimated the changes observed in the present study. In the present study, the individuals in normal hearing group were slightly older than the individuals with ANSD group. The difference in age between the groups could have overestimated the amplitude difference seen in TEOAEs. However, this possibility was clarified by omitting the individuals younger than 16 years of age from the analysis. Nevertheless, analysis revealed that, mean difference in amplitude between the groups was significant for both overall amplitude and narrow band analysis. These observations indicate that the observed difference in amplitude between the groups may not be greatly influenced by the difference in age. In addition, due to the age difference, the normal hearing group might have had a higher risk of music exposure compared to the ANSD group, leading to the reduced amplitude of TEAOEs (Hamdan et al., 2008; Le Page and Murray, 1998). Also, music exposure is thought to have influenced the results, since the individuals with ANSD are known to not appreciate music owing to their impaired auditory perception. Hence, the difference in amplitude between the groups may have also been overestimated by increased music exposure in the normal hearing group. The level of the stimulus used to evoke TEOAEs may also have had an effect on the latency and amplitude of TEOAEs in the current study. The level of the stimulus employed to elicit TEOAEs may have overestimated the difference in latency and amplitude between the groups. According to Guinan et al. (2003) stimulus level (click level) of 70 dB peak SPL, while recording TEOAEs, elicits medial olivocochlear (MOC) efferent activity in normal hearing adults. More specifically, when TEOAEs are elicited by clicks of 70 dB peak SPL used as ipsilateral and/or contralateral elicitors, they are found to suppress the TEOAEs. In the present study, TEOAEs were recorded at 75e80 dB peak SPL (which is the typical stimulus level used in clinical practice) in both the groups. The amount of TEOAE suppression observed in individuals with ANSD is significantly lower than the normal hearing individuals (Starr et al., 2001; Berlin et al., 1993). Thus, assuming that clicks >75 dB peak SPL elicited efferent suppression in normal hearing and the amount of suppression is greater in normal hearing compared to ANSD, the amplitude and latency changes observed in present study, were somewhat overestimated and the true increase being in part hidden by the suppression effect. 5. Conclusion In the present study, it was observed that individuals in ANSD group had significantly higher pure-tone thresholds and significantly greater energy of TEOAEs in comparison to the normal

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hearing group. Also, the timeefrequency analysis of TEOAEs showed that the TEOAEs measured in ANSD group had significantly higher energy and slightly reduced latency compared to those measured in normal hearing group at low and mid frequencies. The increased amplitude and reduced latencies of TEOAEs could be attributed to the efferent system damage, especially at low and mid frequencies seen in individuals with ANSD. The results of the present study emphasize the use of timee frequency analysis of cochlear functionality in the clinical evaluation of individuals affected by ANSD. This would be of particular relevance, especially in individuals with ANSD, because it helps in understanding the etiology and management of ANSD. It has been reported that the TEOAEs may disappear over time in individuals with ANSD (Deltenre et al., 1999; Shoup and Rosser, 2007). This disappearance of TEOAEs has been attributed to underlying pathophysiology (Marshall and Heller, 1996; Tallat et al., 2013) and use of amplification (Berlin et al., 2010). A periodical monitoring of the cochlear function in these individuals by means of timeefrequency analysis of TEOAEs may be helpful in better understanding the patho-physiology leading to cochlear dysfunction. The proposed WT of TEOAEs is not yet completely developed, because of the shortage of normative data available. Hence, further studies would be essential to focus on evaluating and monitoring the cochlear function in individuals with ANSD using timeefrequency analysis of TEOAEs considering the duration of the ANSD condition and use of amplification devices. Acknowledgments The authors acknowledge with gratitude Prof. S. R. Savithri, Director, All India Institute of Speech and Hearing, Mysore for permitting to conduct the study at the institute. We thank two anonymous reviewers’ for their valuable and constructive comments on the earlier version of the manuscript. The authors also like to acknowledge the individuals participated in the study for their co-operation. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.heares.2014.04.005. References Amatuzzi, M., Liberman, M.C., Northrop, C., 2011. Selective inner hair cell loss in prematurity: a temporal bone study of infants from a neonatal intensive care unit. J. Assoc. Res. Otolaryngol. 12, 595e604. Avilala, V.K.Y., Mohan, D., Barman, A., 2012. Spontaneous Otoacoustic emissions in individuals with auditory neuropathy spectrum disorders. Audiol. Med. 10, 50e 54. Berlin, C.I., Hood, L.J., Szabo, W.P., Righy, P.C., Jackson, D.F., 1993. Contralateral suppression of nonlinear click evoked otoacoustic emissions. Hear. Res. 71, 1e11. Berlin, C.I., Hood, L.J., Morlet, T., Wilensky, D., Li, L., Mattingly, K.R., et al., 2010. Multi-site diagnosis and management of 260 patients with auditory neuropathy/Dys-synchrony (Auditory neuropathy spectrum disorder). Int. J. Audiol. 49, 30e43. Berlin, C., Hood, L., Morlet, T., Rose, K., Brashears, S., 2003. Auditory neuropathy/ Dys-synchrony: diagnosis and management. Ment. Retard. Dev. Disabil. Res. Rev. 9, 225e231. Berlin, C.I., Bordelon, J., St John, P., Wilensky, D., Hurleym, A., Kluka, E., Hood, L.J., 1998. Reversing click polarity may uncover auditory neuropathy in infants. Ear Hear. 19, 37e47. Bhagat, S., Bass, J., Qaddoumi, I., Brennan, R., Wilson, M., Wu, J., et al., 2013. Time frequency analysis of transient evoked emissions in children exposed to carboplatin chemotherapy. Audiology Neurotol. 18, 71e82. Carhart, R., Jerger, J.F., 1959. Preferred method for clinical determination of puretone thresholds. J. Speech Hear. Disord. 24, 330e345. Deltenre, P., Mansbach, A.L., Bozet, C., Christiaens, F., Barthelemy, P., Paulissen, D., Renglet, T., 1999. Auditory neuropathy with preserved cochlear microphonics and secondary loss of otoacoustic emissions. Audiology 38, 187e195.

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